Justine is the best on clay, but not by much really. Both Venus and specially Serena have all the tools to be good on clay. The difference between them and Justine is that Justine is contend to go the rally, while specially Serena doesn´t have the patience for that. In ´02 she became more controlled and look what she did during the clay-season. same happened during RG´03...when she was patient, there wasn´t much Justine could do to hurt her.
And no it ain´t just the power, Lindsay has power, Monica has power, Bovina has power..it´s the total package. Defense, offense, great-serving,forehand, backhand all solid: specially for Serena. Really if you think about Serena´s the one player out there who´s got the least weaknesses.